Portable communication devices, such as cellular telephones, personal digital assistants (PDAs) and other communication devices often include multiple radio receivers or transceivers. For example, a cellular telephone may include a cellular transceiver, a television receiver and an FM radio. One of the features included in many FM radio receivers is a function referred to as a “seek” function. The seek function enables the radio to automatically and autonomously search for valid channels (i.e., channels where station broadcast exists). One important aspect of implementing a robust seek function is to have the ability to reliably detect a valid channel.
A valid channel is a channel in which a broadcasting station exists within reception range of the radio receiver, even if the broadcast signal in the channel is very weak. An invalid channel is a channel in which there is no station broadcast. The goal of a seek function is to skip invalid channels and lock on to detected valid channels.
Complicating the design of a robust seek function is that the same radio should work in very different broadcast environments. For example, the same radio design should work in rural areas where signals are weak and the noise floor is low, and in large metropolitan areas where signals are strong, the noise floor is high and where many signals coexist.
One manner of determining whether a station is valid is the detection and measurement of what is referred to as a received signal strength indictor (RSSI). The RSSI signal is a parameter measured at the receiver and is an indicator of the signal strength in the desired, or target, channel. When the RSSI signal is higher than a certain threshold, it often implies the existence of a valid channel, or station, and vice versa.
Unfortunately, there are challenges related to using the RSSI signal as the only criteria to determine whether a valid station is broadcasting in the area of the radio. First, the optimal value of the RSSI threshold varies in different environments. In rural areas, where both the signal strength and the noise floor are low, a small value for the threshold is required. In metropolitan (metro) areas, where both the signal strength and the noise floor are higher than in a rural area, a higher threshold value is more optimal. Unfortunately, the threshold value is not typically user-adjustable. This threshold is predetermined and preset by the radio manufacturer. If the threshold is set too high, the radio may erroneously skip weak, but valid channels during the seek operation. On the other hand, if the threshold is set too low, the radio may erroneously lock on an invalid channel, where the noise energy is higher than the threshold. As a result, trade-offs are typically made in selecting the RSSI threshold value, which compromises the robustness of the seek function.
Another challenge related to using the RSSI signal as the only criteria to determine whether a valid station is broadcasting in the area of the radio is due to the energy spillover from a strong adjacent channel. As a result, the desired channel may contain a significant amount of energy spillover from its adjacent channels and pass the RSSI threshold test, even through there is no station broadcast in the desired channel.
Therefore, it would be desirable to have a robust seek function for a radio receiver that works in a variety of broadcast environments.